The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators like Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin-and angle-resolved photoelectron spectroscopy (ARPES) experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.The hallmark of a topological insulator is a single spinpolarized Dirac cone at the surface which is protected by time reversal-symmetry and originates from a band inversion in the bulk [1,2]. Notably, breaking time-reversal symmetry by magnetic order does not necessarily destroy the non-trivial topology but instead may drive the system into another topological phase. One example is the quantum anomalous Hall (QAH) state that has been observed in magnetically doped topological insulators [3]. The QAH state, in turn, may form the basis for yet more exotic electronic states, such as axion insulators [4,5] and chiral Majorana fermions [6]. Another example is the antiferromagnetic topological insulator state which is protected by a combination of time-reversal and lattice translational symmetries [7].Magnetic order in a topological insulator has mainly been achieved by doping with 3d impurities [3,8], which however inevitably gives rise to increased disorder. By contrast, the layered van der Waals material MnBi 2 Te 4 [9, 10] has recently been proposed to realize an intrinsic magnetic topological insulator [11][12][13][14], i.e. a compound that features magnetic order and a topologically non-trivial bulk band structure at the arXiv:1903.11826v2 [cond-mat.str-el]

and Anna Isaeva (anna.isaeva@tu-dresden.de) ¥ These authors contributed equally to this work. 2 Combinations of non-trivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances material candidates are emerging. Yet, a compound with a band-inverted electronic structure and an intrinsic net magnetization remains unrealized. MnBi2Te4 is a candidate for the first antiferromagnetic topological insulator and the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. For n = 1, we confirm a non-stoichiometric composition proximate to MnBi4Te7 and establish an antiferromagnetic state below 13 K followed by a state with net magnetization and ferromagnetic-like hysteresis below 5 K. Angleresolved photoemission experiments and density-functional calculations reveal a topological surface state on the MnBi4Te7(0001) surface, analogous to the non-magnetic parent compound Bi2Te3. Our results render MnBi4Te7 as a band-inverted material with an intrinsic net magnetization and a complex magnetic phase diagram providing a versatile platform for the realization of different topological phases.Soon after the discovery of topological insulators (TIs) a decade ago 1 , the role of magnetism and its potential to modify the electronic topology emerged as a central issue in the field of topological materials. Magnetic degrees of freedom provide a powerful means of tuning the decisive characteristic of any topological system: its symmetry. By now it is recognized that the interplay between magnetic order and electronic topology offers a rich playground for the realization of exotic topological states of matter, such as the quantum anomalous Hall state, 2,3 the axion insulator state, 4-6 and magnetic Weyl and nodal lines semimetals, 7-9 enabling in turn different routes to spintronic applications. [10][11][12] The non-trivial topology in paradigmatic TIs like Bi2Te3 is a result of band inversion driven by strong spin-orbit interaction. 13,14 Until recently, the interplay with magnetism in this class of systems has been mostly explored by extrinsic methods, such us doping a known TI

Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain‐induced selective symmetry breaking of 3d‐t2g(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application.

We studied the local Ru 4d electronic structure of α-RuCl3 by means of polarization dependent xray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is −12 ± 10 meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a J eff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, α-RuCl3 is an ideal candidate for the realization of Kitaev physics.Geometrically frustrated quantum spin systems are important owing to the fact that frustration often results in a suppression of conventional mean field ground states in favor of more exotic phases of matter. Current research focuses on the effect of spin-orbit coupling (SOC) and the role it plays in the realization of different exotic phases such as unconventional superconductivity or quantum spin liquids [1-3]. Especially, quantum spin liquids can result in topological states with fractional excitations. An important, theoretically solvable model is the Kitaev model with spin-1/2 on a honeycomb lattice, where the coupling between neighboring spins is highly anisotropic with bond-dependent spin interactions. In contrast to spin liquids arising from usual geometrical frustrated spin arrangements, the bond-dependent spin interactions within the Kitaev model frustrate the spin configuration on a single site [4].The search for fractionalized excitations and the identification of a Kitaev spin liquid state has been experimentally quite difficult. Increased attention has been focussed on the honeycomb iridates [5,6], starting from the assumption that large spin-orbit coupling is the leading energy scale in determining the ground state such that the Ir 5d t 2g orbitals are described in terms of J eff = 1/2 and 3/2 orbitals. However, the real iridate systems exhibit trigonal distortion (D trig = 0.1 eV [6]) and a significant itinerant character of the Ir 5d orbitals [7-9], which complicates the electronic ground state. Despite a flurry of both theoretical and experimental studies, the nature of the ground state in honeycomb iridates are being fiercely debated and the occurrence of Kitaev physics is still far from clear.Recently, α-RuCl 3 has been suggested as a promising candidate material for the realization of the Kitaev model [10] and excitations observed via Raman [11,12] and in-elastic neutron scattering [13] have been presented as evidence that α-RuCl 3 may be close to a quantum spin liquid ground state. In the last two years a number of publications discussing the realization of the Kitaev physics in α-RuCl 3 has appeared in literature [3,[14][15][16][17][18][20][21][22][23][24][25][26][27][28][29]...

Graphene-spaced magnetic systems with antiferromagnetic exchange-coupling offer exciting opportunities for emerging technologies. Unfortunately, the in-plane graphene-mediated exchange-coupling found so far is not appropriate for realistic exploitation, due to being weak, being of complex nature, or requiring low temperatures. Here we establish that ultra-thin Fe/graphene/Co films grown on Ir(111) exhibit robust perpendicular antiferromagnetic exchange-coupling, and gather a collection of magnetic properties well-suited for applications. Remarkably, the observed exchange coupling is thermally stable above room temperature, strong but field controllable, and occurs in perpendicular orientation with opposite remanent layer magnetizations. Atomistic first-principles simulations provide further ground for the feasibility of graphene-spaced antiferromagnetic coupled structures, confirming graphene’s direct role in sustaining antiferromagnetic superexchange-coupling between the magnetic films. These results provide a path for the realization of graphene-based perpendicular synthetic antiferromagnetic systems, which seem exciting for fundamental nanoscience or potential use in spintronic devices.